This report describes studies on the biochemical and physiological mechanisms by which cell functions are regulated by hormones, agonists, and in pathophysiological states, including aging. The findings summarize investigations on: 1. Investigations of the Na+-H+ Exchange Mechanism. 2. Thyroid hormone regulation of Na+-H+ exchange activity. 3. Effect of L-Triiodothyronine (T3) on Na+-H+ Exchange Activity in Cultured Kidney (OK) Cells. 4. Renal Na+-H+ Exchange Activity in Aged Rats. 5. Effects of Ovariectomy and Estrogen Replacement on Renal Water and Electrolyte Metabolism in Rats. 6. Dopamine Increases Cytosolic Calcium in Renal Proximal Tubules. 7. Age-Associated Changes in Hormone-Induced Membrane Signal Transduction Systems in Renal Proximal Tubules. 8. Signal Transduction Mechanisms in Murine Splenic Lymphocytes. 9. Bradykinin Induces Changes in Cytosolic Calcium in Human Fibroblasts (IMR-90). 10. Mechanism of the Decrease in Renal Ammonia Excretion After an Acid Load in Aged Rats. 11. Modulation of the Ca2+-activated K+ Channel in Medullary Thick Ascending Limb Cells.